Velocity controlled disk drive head retraction with reduced audible noise

ABSTRACT

A control circuit controls current through a voice coil motor (VCM) during retraction of a transducing head from the surface of a recordable medium. The transducing head is carried on movable member actuated by the VCM. The control circuit includes a measuring circuit for measuring a back electromotive force (back EMF) from the VCM. The control circuit also includes a driver circuit connected to terminals of the VCM for providing the VCM with a drive current having a magnitude based on the back EMF from the VCM. The control circuit further includes a chop clock to alternate between measuring the back EMF from the VCM and providing the VCM with the drive current at a random frequency within a range of frequencies around a median frequency.

BACKGROUND OF THE INVENTION

The present invention relates to disk drives and storage devices. Inparticular, the present invention relates to reducing undesirableaudible noise in velocity controlled hard disk drive head/arm assembliesduring head retraction.

Generally, a magnetic hard disk drive (HDD) includes a magneticread/write head and several magnetic disks, each disk having concentricdata tracks for storing data. The disks are mounted on a spindle motor,which causes the disks to spin. The read/write head is typically mountedon a slider, which is mounted to a suspension or load beam. The loadbeam is attached to an actuator arm of an actuator, which moves theread/write head over the spinning disk during operation. As the disksspin, the slider suspended from the actuator arm “flies” a smalldistance above the disk surface. The slider carries a transducing headfor reading from or writing to a data track on the disk.

In addition to the actuator arm, the slider suspension comprises abearing about which the actuator arm pivots. A large scale actuatormotor, such as a voice coil motor (VCM), is used to move the actuatorarm (and the slider) over the surface of the disk. When actuated by theVCM, the actuator arm can be moved from an inner diameter to an outerdiameter of the disk along an arc until the slider is positioned above adesired data track on the disk.

A control circuit is coupled to a coil in the VCM in order tocontrollably supply current to the coil. When a current is passedthrough the coil, a motive force is exerted on the actuator arm. Theactuator arm is subjected to a force tending to accelerate the actuatorarm at a rate defined by the magnitude of the current, and in adirection defined by the polarity of the current. Thus, in order toaccelerate or decelerate the actuator arm until it is moving at adesired velocity and in a desired direction, it is important to know theactual direction and velocity of the actuator arm. It is known that theback electromotive force (back EMF) from the coil of the actuator isrepresentative of the velocity and direction of movement of the actuatorarm.

Parking zones in an HDD allow the read/write head to be safely landedafter the hard drive has ceased operation. When an HDD is powered down,it usually performs certain operations before actually disconnectingfrom the external power source. One of these power down operations is tooperate the actuator arm to move the head to the parking zone. If thehead is not moved to the parking zone prior to power down, the head willland on the disk after the disk stops spinning, potentially damaging thedisk and the read/write head.

In many conventional systems, the drive voltage or current to the VCM iscontinuously enabled and disabled at a constant frequency in order toprovide alternating driving of the VCM (enabled) and measuring of theactuator arm speed by the control circuitry (disabled). The frequency ofthe drive voltage or current in these systems is typically at afrequency within the audible range, thereby causing an undesirableconstant tone during head retraction. One method of resolving thisproblem is to limit the magnitude of the drive to the VCM to keep theacoustic noise level to a minimum. However, by limiting the maximumdrive to keep the acoustic level low, the speed of the retraction cannotbe controlled as easily. This is especially true when the read/writehead reaches the parking zone or when a retraction magnet acceleratesthe actuator arm.

Thus, there is a need for an actuator arm retract controller thatreduces audible noise while still allowing velocity control of theactuator arm during retraction.

BRIEF SUMMARY OF THE INVENTION

The present invention is a control circuit for controlling currentthrough a voice coil motor (VCM) during retraction of a transducing headfrom the surface of a recordable medium. The transducing head is carriedon movable member actuated by the VCM. The control circuit includes ameasuring circuit for measuring a back electromotive force (back EMF)from the VCM. The control circuit also includes a driver circuitconnected to terminals of the VCM for providing the VCM with a drivecurrent having a magnitude based on the back EMF from the VCM. Thecontrol circuit further includes switching circuitry to aperiodicallyand alternately enable the measuring circuit and the driver circuit.

In one embodiment, the switching circuitry comprises a random frequencygenerator including a control circuit clock input and a random clockoffset input. The random clock offset input is preferably provided by aprogrammable register, such as a linear feedback shift register. Theswitching circuitry alternately enables the measuring circuit and thedriver circuit at a random frequency within a range of frequenciesaround a median frequency.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a typical velocity controlled disk drivehead retraction control system.

FIG. 2 is a schematic diagram of a velocity controlled disk drive headretraction control system with audible noise reduction according to thepresent invention.

FIG. 3 is a schematic diagram of a chop clock generator according to anembodiment of the present invention.

DETAILED DESCRIPTION

FIG. 1 is a block diagram of typical velocity controlled disk drive headretraction control system 10. Disk drive head retraction control system10 includes control block 12, motor driver block 14, motor 16, actuatorarm 18, transducing head 20, disk 22, parking location 23, and backelectromotive force (back EMF) sampling block 24. Control block 12receives a velocity demand signal as an input and provides a controlsignal to motor driver block 14. Motor driver block 14 is connected tomotor 16 and provides a drive current to motor 16 to move actuator arm18. Back EMF sampling block 24 is connected to motor 16 to sample theback EMF from motor 16. Back EMF sampling block 24 provides a signal tocontrol block 12.

In normal operation, a drive current is provided to motor 16 to actuateactuator arm 18. When actuated by motor 16, actuator arm 18 can be movedfrom an inner diameter to an outer diameter of disk 22 along an arcuntil transducing head 20 is positioned above a desired data track ondisk 22. Disk 22 includes a plurality of concentric tracks on which dataand position information is recorded. Disk 22 is mounted on a spindlemotor, which causes disk 22 to spin. Transducing head 20 suspended fromactuator arm 18 flies above the surface of disk 22 as it spins.Transducing head 20 is operable to read the data and positioninformation from tracks of disk 22 and generate an input signalrepresentative of the data and position information.

When a disk drive is powered down, it usually performs certainoperations before actually disconnecting from the external power source.One of these power down operations is to operate actuator arm 18 to movetransducing head 20 to parking location 23. Parking location 23 allowstransducing head 20 to be safely landed after the disk drive has ceasedoperation. Parking location 23 is located at the outermost edge of disk22 and typically includes a ramp to raise transducing head 20 and parkit off of disk 22 in an elevated position. If the head is not moved toparking location 23 prior to power down, the head will land on disk 22after disk 22 stops spinning, potentially damaging disk 22 andtransducing head 20.

In the event of a catastrophic shut down (i.e., external power issuddenly removed), there is no external power to perform power downprocedures, including moving transducing head 20 to parking location 23.Typically, the momentum of the spinning disk operates the spindle motorto generate a back electromotive force at the motor terminals, which isrectified and stored to provide power to disk drive head retractioncontrol system 10 upon a catastrophic shut down. However, the poweravailable to power motor 16 is limited by the spindle motor resistanceand the back electromotive force of the spindle motor.

Disk drive head retraction control system 10 is a typical implementationof a system to retract transducing head 20 in a catastrophic power losssituation. Control system 10 alternately drives motor 16 with a drivecurrent during a driving phase and measures the back EMF from the motor16 during a measuring phase. The back EMF from motor 16 isrepresentative of the velocity and direction of movement of actuator arm18.

During the driving phase, control block 12 receives a velocity demandsignal representing a preferred retraction velocity and direction oftransducing head 20. The velocity demand signal is typically aprogrammable value stored in a register or other storage device. Thetorque load encountered by transducing head 20 as it traverses its pathin a retract operation varies considerably with position. Consequently,control block 12 must constantly adjust the drive current to transducinghead 20 to correspond to the velocity demand signal. Control block 12provides a signal to motor driver block 14 corresponding to the drivecurrent necessary to adjust the actual velocity of the transducing head20 to correspond to the preferred velocity of the velocity demandsignal. Motor driver block 14 amplifies this signal and drives motor 16with a drive current to accelerate or decelerate retraction oftransducing head 20 toward parking location 23.

During the measuring phase, the drive current to motor 16 is disabled.Subsequently, the back EMF from motor 16 is sampled by back EMF sampleblock 24. Preferably, the back EMF from motor 16 is sampled a pluralityof times during the measuring phase and averaged to provide an averageback EMF signal. The back EMF signal is then amplified by back EMFsample block 24 and passed to control block 12. This signal representsthe actual velocity of transducing head 20. Control block 12 then usesthe sampled back EMF signal to compare the actual velocity oftransducing head 20 with the preferred velocity of the velocity demandsignal. The driving phase then begins again as control block 12 providesa signal to motor driver block 14 corresponding to the drive currentnecessary to adjust the velocity of transducing head 20 to correspond tothe preferred velocity of the velocity demand signal.

In many conventional systems, the drive voltage or current to motor 16is continuously enabled and disabled at a constant frequency in order toalternately drive motor 16 (enabled) and measure the velocity ofactuator arm 18 (disabled). The frequency of alternation betweenenabling and disabling of the drive voltage or current in these systemsis typically at a frequency within the audible range (i.e., 20 Hz –20kHz), thereby causing an undesirable constant tone during headretraction.

FIG. 2 is a schematic diagram of velocity controlled disk drive headretraction control system 50 with audible noise reduction according toan embodiment of the present invention. Retraction control system 50includes the same functional blocks as shown and described with regardto FIG. 1, including control block 12, motor driver block 14, motor 16,and back EMF sample block 24. Each of these blocks is shown in moredetail in FIG. 2. Control block 12 includes chop clock generator circuit52 and control circuit 54. Motor driver block 14 includes poweramplifier circuit 56. One input of power amplifier circuit 56 isconnected to an output of control circuit 54, while a second input ofpower amplifier circuit 56 is connected to a reference voltage V_(Ref).The output of chop clock generator circuit 52 is connected to atri-state input of power amplifier circuit 56. The outputs of poweramplifier circuit 56 are connected across motor 16. Back EMF sampleblock 24 includes back EMF amplifier 60 and analog-to-digital converter(ADC) 62. The inputs of back EMF amplifier 60 are connected across motor16, and the output of back EMF amplifier 60 is provided to the input ofADC 62. The output of ADC 62 is provided to control circuit 54.

In operation, retraction control system 50 alternates between a drivingphase and a measuring phase to control retraction of transducing head20. During the driving phase, control circuit 54 receives a velocitydemand signal representing a preferred retraction velocity anddirection. Control circuit 54 subsequently provides a signal to poweramplifier circuit 56 corresponding to the drive current necessary toadjust the actual velocity of the transducing head to coincide with thepreferred velocity of the velocity demand signal. Power amplifiercircuit 56 amplifies this signal and drives motor 16 with a drivecurrent to accelerate or decelerate retraction of the transducing head.

During the measuring phase, the drive current to motor 16 is disabled,and the back EMF from motor 16 is sampled and amplified by back EMFamplifier 60. In one embodiment, multiple back EMF measurements aretaken and averaged during each measuring phase to improve the accuracyof the back EMF measurement. The analog back EMF signal is then passedto ADC 62 to produce a digital representation of the back EMFmeasurement. The digital back EMF measurement is then passed to controlcircuit 54. This signal represents the actual velocity of transducinghead 20. Control circuit 54 then uses the sampled back EMF signal tocompare the actual velocity of transducing head 20 with the preferredvelocity of the velocity demand signal. In one embodiment, controlcircuit 54 includes a digital implementation of a proportional integral(PI) controller to perform this operation. The driving phase then beginsagain as control circuit 54 provides a signal to power amplifier 56corresponding to the drive current necessary to adjust the velocity oftransducing head 20 to coincide with the preferred velocity of thevelocity demand signal.

Chop clock generator 52 controls alternating between the driving phaseand the measuring phase in retraction control system 50. Chop clockgenerator 52 preferably produces a signal that comprises a rectangularwave. The output of chop clock generator 52 is connected to thetri-state input of power amplifier circuit 56 and to the enabling inputof ADC 62. Power amplifier circuit 56 is enabled when its tri-stateinput is low and ADC 62 is enabled when its enabling input is high.Thus, when the output of chop clock generator 52 transitions to the lowstate of the rectangular wave, power amplifier circuit 56 is enabled,ADC 62 is disabled, and the driving phase commences. Conversely, whenthe output of chop clock generator 52 transitions to the high state ofthe rectangular wave, ADC 62 is enabled, power amplifier circuit 56 isdisabled, and the measuring phase commences.

Chop clock generator 52 has two inputs, system clock 70 and nominalperiod 72. System clock 70 provides a recurring rectangular or squarewave at a constant frequency to clock all circuit components of chopclock generator 52. System clock 70 is typically provided by anoscillator internal to retraction control system 50.

Nominal period 72 determines the median period of the output signal fromchop clock generator 52. In one embodiment, nominal period 72 isprovided by a programmable control register, such as a linear feedbackshift register (LFSR). Chop clock generator 52 uses nominal period 72 toproduce an aperiodic chop clock signal. As a result, retraction controlsystem 50 is switched between the driving phase and the measuring phaseat a random frequency within a range of frequencies around the medianfrequency. The random frequency of the chop clock signal is preferablyin a range of frequencies within about 20 or 30 percent of the medianfrequency. When retraction control system 50 is switched between thedriving phase and the measuring phase at a random frequency within thisrange of frequencies, the audible noise that typically occurs duringretraction of the transducing head is significantly reduced. Morespecifically, in conventional head retraction systems, a constantaudible tone is heard during head retraction after a catastrophic powerloss since alternating between the driving phase and the measuring phaseoccurs at a constant frequency within the audible frequency range. Inretraction system 50 according to the present invention, thisalternating occurs at various frequencies that are spread out around themedian frequency, thus eliminating the constant tone and reducing theoverall audible noise during retraction of the transducing head.

FIG. 3 shows one embodiment of chop clock generator 52. It will beappreciated that chop clock generator 52 as described below is merelyexemplary, and any device capable of producing a signal with a randomfrequency within a range of frequencies around a median frequency may beused in place of chop clock generator 52 without departing from thespirit and scope of the present invention.

Chop clock generator 52 includes random number generator 100, scalingblock 102, table lookup block 104, adder 106, divider 108, comparator110, flip flop 112, up/down counter 114, and comparator 116. Randomnumber generator 100 receives system clock 70 and the output ofcomparator 116 as inputs and provides an output to an input of scalingblock 102. Nominal period 72 is provided as an input to scaling block102 and table lookup block 104. The outputs of scaling block 102 andtable lookup 104 are provided to adder 106, and the output of adder 106is provided to divider 108. Divider 108 supplies one input to comparator110. Up/down counter 114 is clocked by system clock 70 and is activatedby an enable retract signal (EN_RETRACT). The output of up/down counter114 is provided to an input of comparator 110 and to an input ofcomparator 116. The output of comparator 116 is provided to the enablinginput of random number generator 100 and the clear control input of flipflop 112. The set control input of flip flop 112 is provided by theoutput of comparator 110. The output of flip flop 112 provides theoutput of chop clock generator 52 (CHOP_CLK), which also provides theenabling input for comparators 110 and 116 and an input of up/downcounter 114.

Nominal period 72 is provided by a 3-bit programmable control registerin one embodiment. This 3-bit value is provided to scaling block 102 andtable lookup 104. Table lookup 104 contains eight programmable 16-bitperiod values. Table lookup 104 is used to convert the 3-bit controlregister value of nominal period 72 to one of the eight 16-bit periodvalues stored in table lookup 104. The 16-bit output of table lookup 104is provided to adder 106.

Scaling block 102 uses the 3-bit value of nominal period 72 to scale theoutput of random number generator 100. The scale factors in scalingblock 102 are chosen to randomize the frequency output of chop clockgenerator 52 in a range of frequencies within about 20 or 30 percent ofthe median frequency. In one embodiment, random number generator 100 isa linear feedback shift register (LFSR) that is updated once per chopclock cycle such that each chop clock period has high time and low timeof equal lengths. The output of scaling block 102 is a 16-bit signedrandom number that is added to the output of table lookup 104 by adder106. The output of adder 106 is a signal with a randomized period basedon nominal period 72. This randomized period is divided by two bydivider 108 and provided to comparator 110.

Up/down counter 114 is clocked by system clock 70 and provides an outputto an input of comparator 110 that counts from zero up to a value equalto nominal period 72 plus the output of scaling block 102 divided by two(that is, the value provided to input A of comparator 110) and back downto zero at a rate defined by system clock 70. Up/down counter 114 beginscounting when the retraction sequence is enabled by asserting theEN_RETRACT input of up/down counter 114. The output of comparator 110provides the set control input of flip flop 112. When up/down counter114 is counting up, the output of chop clock generator 52 is set high.Conversely, when up/down counter 114 is counting down, the output ofchop clock generator 52 is set low. The output of chop clock generator52 is provided to a decrementing control input of up/down counter 114and the enabling input of comparator 116. The complement of the outputfrom chop clock generator 52 is provided to the enabling input ofcomparator 110. Thus, when the output chop clock generator 52 is sethigh, the enabling input of comparator 116 is asserted and the enablinginput of comparator 110 is de-asserted. Also, the decrementing controlinput of up/down counter 114 is asserted, which causes up/down counter114 to begin counting down. This causes the output of chop clockgenerator 52 to switch to low, which asserts the enabling input ofcomparator 110, de-asserts the enabling input of comparator 116, andde-asserts the decrementing input of up/down counter 114. As a result,up/down counter 114 starts counting up, which causes the output of chopclock generator 52 to switch to high. The switching of the output ofchop clock generator 52 between high and low occurs aperiodically at afrequency defined by the output signal of adder 106 (as describedabove).

In summary, in many conventional systems the drive voltage or current tothe VCM is continuously enabled and disabled at a constant frequency inorder to provide alternating driving of the VCM (enabled) and measuringthe actuator arm speed by the control circuitry (disabled). Thefrequency of the drive voltage or current in these systems is typicallyat a frequency within the audible range, thereby causing an undesirableconstant tone during head retraction. The present invention is a controlcircuit for controlling current through a voice coil motor (VCM) withreduced audible noise during retraction of a transducing head from thesurface of a recordable medium. The transducing head is carried on amovable member actuated by the VCM. The control circuit includes ameasuring circuit for measuring a back electromotive force (back EMF)from the VCM. The control circuit also includes a driver circuitconnected to terminals of the VCM for providing the VCM with a drivecurrent having a magnitude based on the back EMF from the VCM. Thecontrol circuit further includes switching circuitry to aperiodicallyand alternately enable the measuring circuit and the driver circuit.

Although the present invention has been described with reference topreferred embodiments, workers skilled in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope of the invention. Particularly, while some forms of theinvention are described in the form of discrete devices, it isrecognized that the circuit is preferably reduced to practice in theform of an integrated circuit (IC). Therefore, terms such as “device”and the like should be construed in their broadest contexts to includeportions of ICs that are conveniently described as functionalcomponents, as well as discrete devices. Likewise, some forms of theinvention are described in terms of logic gates and chips that couldalso be implemented by discrete devices, all within the scope and spiritof the present invention.

1. A control device for controlling current through a voice coil motor(VCM) during retraction of a transducing head from the surface of arecordable medium, the transducing head carried on a movable memberactuated by the VCM, the control device comprising: a measuring circuitfor determining an average back electromotive force (back EMF) from theVCM by measuring the back EMF a plurality of times and averaging themeasured back EMF, wherein the average back EMF from the VCM is relatedto a velocity of the movable member; a driver circuit connected to theVCM for providing the VCM with a drive current having a magnitude basedon the average back EMF from the VCM; and switching circuitry thataperiodically and alternately enables the measuring circuit and thedriver circuit.
 2. The control device of claim 1, wherein the switchingcircuitry comprises a random frequency generator including a systemclock input and a nominal period input.
 3. The control device of claim1, wherein the switching circuitry alternately enables the measuringcircuit and the driver circuit at a random frequency within a range offrequencies around a median frequency.
 4. The control device of claim 3,wherein the random frequency is derived from a programmable register. 5.The control device of claim 4, wherein the programmable registercomprises a linear feedback shift register.
 6. The control device ofclaim 3, wherein the range of frequencies is within about 20 percent ofthe median frequency.
 7. The control device of claim 3, wherein therange of frequencies is within about 30 percent of the median frequency.8. The control device of claim 3, wherein the median frequency isbetween about 100 Hertz (Hz) and 5 kilohertz (kHz).
 9. The controldevice of claim 1, and further comprising: a microcontroller forcalculating the velocity of the movable member based on the average backEMF from the VCM and for controlling the magnitude of the drive currentprovided by the drive circuit based on the velocity of the movablemember.
 10. The control device of claim 1, wherein the control device isfabricated in an integrated circuit.
 11. A method of controllingretraction of a transducing head from the surface of a recordable mediumwith reduced audible noise, the transducing head carried on a movablearm actuated by an electric motor, the method comprising: determining anaverage back electromotive force (back EMF) from the electric motor bymeasuring the back EMF a plurality of times and averaging the measuredback EMF; driving the electric motor with a drive current having amagnitude based on the average back EMF from the electric motor; andaperiodically and alternately determining the average back EMF from theelectric motor and driving the electric motor with a drive current. 12.The method of claim 11, wherein the aperiodic alternation is performedat a random frequency within a range of frequencies around a medianfrequency.
 13. The method of claim 12, wherein the range of frequenciesis within about 20 percent of the median frequency.
 14. The method ofclaim 12, wherein the range of frequencies is within about 30 percent ofthe median frequency.
 15. The method of claim 11, wherein aperiodicallyand alternately determining the average back EMF from the motor anddriving the electric motor with a drive current comprises: producing afrequency offset; adjusting a median frequency by the frequency offsetto produce a random frequency; and alternately determining the averageback EMF from the electric motor and driving the electric motor with adrive current at the random frequency.
 16. The method of claim 15,wherein producing the frequency offset comprises generating a randomvalue derived from a programmable register.
 17. The method of claim 16,wherein the random value is generated with a linear feedback shiftregister.
 18. The method of claim 11, wherein driving the electric motorwith a drive current having a magnitude based on the average back EMFcomprises: calculating the velocity of the moveable arm based on theback EMF; and controlling the magnitude of the drive current provided bythe drive circuit based on the velocity of the movable arm.
 19. Acircuit for controlling retraction of a device carried on a movablemember actuated by an electric motor, the retraction system havingreduced audible noise and comprising: a measuring circuit fordetermining an average back electromotive force (back EMF) from theelectric motor by measuring the back EMF a plurality of times andaveraging the measured back EMF; a driver circuit for providing themotor with a drive current having a magnitude based on the average backEMF from the motor; and switching means for aperiodically andalternately enabling the measuring circuit and the driving circuit. 20.The circuit of claim 19, wherein the switching means alternately enablesthe measuring circuit and the driver circuit at a random frequencywithin a range of frequencies around a median frequency.
 21. The circuitof claim 20, wherein the range of frequencies is within about 20 percentof the median frequency.
 22. The circuit of claim 20, wherein the rangeof frequencies is within about 30 percent of the median frequency. 23.The circuit of claim 20, wherein the random frequency is derived from aprogrammable register.
 24. The circuit of claim 23, wherein theprogrammable register comprises a linear feedback shift register. 25.The circuit of claim 19, and further comprising: a microcontroller forcalculating the velocity of the movable member based on the average backEMF and for controlling the magnitude of the drive current provided bythe drive circuit based on the velocity of the movable member.